Devonian Terrestrial Tetrapods – Further Information
The article published on the Polish paper providing details on the trace fossils indicating that Devonian creatures were venturing onto land many millions of years earlier than previously thought has attracted a lot of comments and additional information.
To read the original article: From the Water onto the Land Thirty-Five Million Years Earlier.
Curator of Vertebrate Palaeontology, Professor Jennifer Clack at the University Museum of Zoology (Cambridge), has an extensive knowledge of early tetrapods and has published a number of papers and books on this particular subject. In 2002, she wrote “Gaining Ground – The Origin and Evolution of Tetrapods”, so Professor Clack is well placed to comment on the Polish trace fossils; she comments.
The timing of the origin of terrestriality is a bit contentious because it depends on what you classify as actually being “terrestrial”. The trackways found in the Polish quarry are certainly older than other previously discovered fossil evidence. The trackways are 18 million years older than the “near-tetrapods” that have been seen as the closest relatives of animals with limbs bearing digits. If the trackways are to be believed, digits were present way back in the early Middle Devonian. In fact, the figure of 35 million years is a bit misleading, because some already known trackways (other trace fossils) pre-date known tetrapods with digited limbs: 18 million years is the safer figure.
There have been numerous theories as to why tetrapods left the water, over the past couple of decades as more material has been found. If the new trackway data is corroborated by body fossils, it could suggest new ideas and render some of the previous theories put forward obsolete.
With body fossils to back up the trackway finds, the story could go either way. We either have the story more or less right, just the timing wrong and the beasts we already know in fact have a long, “ghost” lineage – or they could change all of our conceptions about what an early tetrapod ancestor looked like and how it was related to later ones.
Until some of these questions are resolved, there is no way really to know why the limbs became capable of supporting weight on land.
We are very grateful to Professor Clack for her input on this complex and fascinating subject. She informs us that work is currently going on an updated and revised edition of her book which deals with the evolution of the tetrapods “Gaining Ground”. One of the great things about palaeontology is that one new discovery can turn conventional, accepted theory on its head, or in this case should that be on its feet!